McClelland anu Nolan — Ions Produced through Mercury. 27 



This is the mobility of the large ions found in the atmosphere and of 

 the large ions in liame-gas which has been allowed to cool. The slowest 

 ion in air which has bubbled through mercury reaches this same mobility 

 after some time, and apparently does not suffer any further decrease of 

 mobility. 



The curves for positive and negative ions were similar, and the slowest 

 ion is the same in the two cases. 



■ In addition to the ions of mobility -UOOo-f, it was evident from the bends 

 in the curve that other distinct groups were present. The amount of 

 the ionization was, however, small, especially when time was given for steady 

 mobilities to be reached, and we endeavoured to increase the ionization before 

 making a detailed examination of the curves. 



The amount of the ionization depended on the manner in which the 

 mercury was thrown about by the air-blast; but when the disturbance is 

 rather violent, it is difficult to obtain steadiness. After trying various types 

 of nozzles we used a turned-up nozzle a very short depth below the surface of 

 the mercury, and placed an iron plate just above the surface, so that the 

 mercury was blown against the plate. This arrangement we found gave a 

 very large supply of positive ions, but the number of negative ions was practi- 

 cally unchanged by the presence of the iron plate. We afterwards used this 

 arrangement, and most of the measurements were made with positive ions, 

 as their greater number made the observations easier and more accurate. 



The mercury was purified well when beginning the observations, but the 

 constant bubbling of air through it brought out further traces of impurities, 

 and these were removed at later stages. It was noticed that the ionization 

 decreased as the mercury became purer. 



We decided to determine carefully the mobilities of the different groups of 

 ions in the following cases : — (1) with undried air and giving sufficient time 

 to allow the mobilities to reach their minimum values ; (2) with dried air and 

 allowing the same time to elapse between the production of the ions and their 

 measurements; (3) and (4) with undried and dried air respectively, the 

 measurements of mobility being carried out as soon after the bubbling as 

 convenient. 



Undried Air. Constant Mobilities. 



The curves on figs. 2 and 3 are examples of those we plotted when the 

 time interval between the mercury vessel and the measuring-tube was 

 sufficient to allow the mobilities to reach steady values. In both figures 

 the curves are plotted in two parts, the smaller numbers on the axis of 

 voltage referring to the lower curve. The rate of air-current through the 



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